The essential parts of a conventional centrifugal pump comprises a rotating member with blades or vanes referred to in the art as an impeller and a housing or casing to surround it. The centrifugal pump depends upon centrifugal forces or a variation of pressure due to the rotation of the impeller. The discharge of the fluid from a centrifugal pump is relatively smooth and steady and can handle various liquids and liquids containing solids such as sand, gravel and stones of various types, of moderate size. Centrifugal pumps are classified as volute type or diffuser type. In the diffuser type there is provided a series of fixed vanes for receiving the fluid discharged from the impeller so as to reduce the velocity of the received fluid by decreasing the Kinetic energy of the fluid stream and converting it to static pressure in the diffuser. It has been found that the centrifugal pump is best suited for producing relatively high pressure and low rates of flow permitting pressure regulation of the pumps.
Vertical cryogenic submerged motor pumps are commonly employed in the liquefied cryogenic gas industry. To this end, they are most prominent in the liquid hydrocarbon industry for liquefied natural gas, LNG, liquefied ethane gas, and liquefied propane and butane gas. There are two categories of LNG pumps that may be classified as the basis of their locations such as In-tank type versus vessel mounted or canned pumps. These categories are described and illustrated in the text entitled “LNG: Basics of Liquefied Natural Gas on pages 64 and 65 discussing “LNG Pumps” published by the University of Texas at Austin in 2007. The In-tank and marine style of the pumps sit at the bottom of large 40 to 60 meter tall cryogenic liquid storage tanks. To mount these pumps in the tall tanks, a 40 to 60 meter pipe column or mast arrangement is applied inside where the pump sits. To keep costs low, it is imperative to keep the pipe column size small and therefore reduce the radial size of the pump's geometry. This fact drives column mounted cryogenic pumps to use axial style diffusers. The prior art generally utilizes a radial vane diffuser downstream of the centrifugal pump. The geometric space restrictions of such an installation dictate the use of axial vane diffusers to save radial space and hence column size and costs.
The function of the diffuser in combination with the centrifugal pump is to efficiently convert the Kinetic energy of the fluid stream from the pump's impeller to pressure energy. The diffuser element is a critical fluid element in the pump responsible for approximately 20-40% of the pump head generated. It is known in the art that the diffuser has substantial influence on the shape of the head curve as the liquid flow rate is varied. Some axial type diffusers utilized with an inducer are known to exhibit unstable or somewhat flat head curves, some exhibit instabilities or flow regions where the slope of the head curve is positive. Pressure regulation is a common means of controlling the centrifugal pump and to be effective it is necessary to have a stable, continuously rising to shut off, head curve for many cryogenic pump applications. Accordingly, when a pump exhibits a head curve with such instability it is useful to manipulate the axial head curve to bring it within a stable, continuously rising curve to shut off to permit pressure regulation of the pump.
Various attempts to improve the shape of the flow-head curve in centrifugal pumps have previously been made. Recent research has revealed that the unstable fluid flow in the pumps is due to the generation of a vortex that resides in the cross-over path located downstream of the pump impeller discharge at the location where the flow direction is changing by being bent for entry into the axial diffuser. A prior art search has revealed various prior art patents attempting to improve the flow-head characteristic of the centrifugal pump. A basic German publication has a brief discussion of radial diffusers with and without return vanes. The article discusses pumps with one stage, the diffuser discharge into a spiral shaped housing or into a ring shaped housing. Multistage pumps are integrated with return vanes that guide the fluid to the next stage, with the exception of the last stage of the multistage pumps.
U.S. Pat. No. 4,981,414 of H. E. Sheets is entitled Method and Apparatus For Producing Fluid Pressure and Controlling Boundary Layer. This patent is directed to turbo machinery having cascade type blades as used on a compressor, blower or turbine and the like. The article describes pressurization by use of a stationary cascade and controlling incentive angle. It does not appear that a solution for surge, span stall, stall cell and inception stall is disclosed.
U.S. Pat. No. 5,286,162 of J. P. Veres is directed to a method of reducing Hydraulic Instability for a centrifugal, volute type pump and compressor by the addition of bleed holes at the volute tongue of the casing of the pump for controlling boundary layer, as Illustrated in FIG. 1.
The search also revealed the disclosure of a centrifugal pump with an improved Axial Diffuser in U.S. Pat. No. 5,330,318 of Ogawa. The teachings are directed to the selection of a specific incident angle of the axial diffuser vanes. It does not appear that this will eliminate unstable conditions sought by the Applicant herein.
U.S. Pat. No. 5,383,764 discloses a diffuser pump having vane blades constructed in two sections for eliminating secondary flow between the sections. This structure may not eliminate an unstable condition.
U.S. Pat. No. 6,923,621 discloses a diffuser for a Turbo pump for suppressing degradation in efficiency while preventing the diffuser from stalling a turbo pump. This is similar in structure to the disclosure in U.S. Pat. No. 5,383,764 and involves an opening at a selected location of a vane to control boundary and prevent separation or a vortex.
U.S. Pat. No. 6,695,579 discloses a diffuser having a variable blade height by providing a flow section profile with increasing cross-section area to cause uneven flow velocity and cause secondary flow in the channel. Totally different approach from subject invention.
U.S. Pat. No. 6,699,008 of D. Jopikse discloses a Flow Stabilizing Device. The disclosure utilizes a vaneless diffuser and provides a slot in front of the inducer and flow back to the inlet in order to eliminate a vortex at a certain flow. This flow is introduced to the outlet of the impeller in an attempt to flush the vortex generated at the cavity between the impeller and diffuser. The disclosure cannot generate a high pressure larger than that generated by the impeller. The flow direction is reverse direction from that of the impeller but the disclosure drawing incorrectly shows the reverse flow.
U.S. Pat. No. 6,514,034 of Okamura et al discloses a pump which is small-sized without increasing the RPM of the impeller while suppressing the unstable portion of the pump head curve due to separation and/or stalling within the region of low flow rate by the provision of a number of grooves in a direction of the pressure gradient of the fluid. This technique is applicable to an axial impeller a device not utilized in the Applicant's invention.
U.S. Pat. No. 6,736,594 is similar to the above described U.S. Pat. No. 6,514,034 by the provision of a slot in the casing that is adjustable so that the recirculation is eliminated.